Convection over Midlatitude Mountains: Insights from the World Wide Lightning Location Network

The western (windward) slopes of extratropical mountains range receive copious amounts of precipitation since synoptic-scale ascent during the passage of midlatitude cyclones is enhanced by forced ascent of moisture-laden air parcels upstream of the mountain. Examples of such setting include the coastal mountains of northwestern North America, the austral Andes in South America, the Southern Alps in New Zealand, the Scandinavian mountains and the coastal range in Japan. The midlatitude cyclones are dominated by clouds and precipitation of stratiform nature, with rather weak vertical air velocities (≤ 2m/s), and accompanied by cold conditions. Likewise, the orographic enhancement is often assumed to occur under stable conditions. Consistently, conventional wisdom suggests little—if any—lightning activity over the cool, hyper-humid midlatitude mountain ranges, especially during the winter semester.

Several lines of evidence, however, suggest that lightning over midlatitude mountain ranges does occur rather frequently, in connection with deep convection, producing high rain rates and eventually hydro-meteorological hazards. In this presentation we explore the spatial distribution of lightning over several extratropical mountains by taking advantage of the global coverage, multi-year World Wide Lightning Location Network (WWLLN) dataset.  We also use atmospheric reanalysis and satellite imagery to characterize the synoptic environment of lightning-producing storms.

We found that lightning tends to occurs during post-frontal conditions, when strong winds impinging the mountains and midlevel cooling produced weakly unstable condition. The strokes tends to cluster spatially depending on the topographic setting.  In the case of the austral Andes, the main cordillera is located a few hundred of km inland and the strokes are more prevalent over the first orographic corrugations (as low as 100 m high) right at the coast with few strokes occurring over the Andes. In contrast, the southern Alps rises sharply within a few tens of kms from the coast and lighting concentrate in that slope. Depending on the amount of convective instability, disperse lightning activity can also occur offshore.